Zig 中的 child process 簡介

在工作中，難免會去重複執行某些某個任務，直到其成功爲止。比如下載依賴，之前我的做法是用 Shell

while ! do something; do echo retry; done

這樣雖然可以，但是有些麻煩，於是就想能不能實現個 cli，來替我們執行，如果用 C 實現的話，可以預想到的，就是 fork + exec，貌似也不是很麻煩。

但是如果要在主進程中，等待 child 完成的之前，收集 child 的 stdout，而不堵塞主進程，那就要採用異步的 API 來輪訓 stdout，比如 select 或 poll， Zig 裏面有個 child_process.zig[1]，它裏面把 fork + exec + poll 都封裝好了，直接使用即可，一些關鍵函數：

/// Collect the output from the process's stdout and stderr. Will return once all output
/// has been collected. This does not mean that the process has ended. `wait` should still
/// be called to wait for and clean up the process.
///
/// The process must be started with stdout_behavior and stderr_behavior == .Pipe
pub fn collectOutput(
    child: ChildProcess,
    stdout: *std.ArrayList(u8),
    stderr: *std.ArrayList(u8),
    max_output_bytes: usize,
) !void {
    debug.assert(child.stdout_behavior == .Pipe);
    debug.assert(child.stderr_behavior == .Pipe);

    // we could make this work with multiple allocators but YAGNI
    if (stdout.allocator.ptr != stderr.allocator.ptr or
        stdout.allocator.vtable != stderr.allocator.vtable)
        @panic("ChildProcess.collectOutput only supports 1 allocator");

    var poller = std.io.poll(stdout.allocator, enum { stdout, stderr }, .{
        .stdout = child.stdout.?,
        .stderr = child.stderr.?,
    });
    defer poller.deinit();

    while (try poller.poll()) {
        if (poller.fifo(.stdout).count > max_output_bytes)
            return error.StdoutStreamTooLong;
        if (poller.fifo(.stderr).count > max_output_bytes)
            return error.StderrStreamTooLong;
    }

    stdout.* = fifoToOwnedArrayList(poller.fifo(.stdout));
    stderr.* = fifoToOwnedArrayList(poller.fifo(.stderr));
}

這是收集 stdout 的，創建進程的地方在 spawnPosix 中，感興趣的可以自行查看，後來有機會再來介紹。

推薦閱讀

microHOWTO: Capture the output of a child process in C[2]
Forking is Cool, or: A Unix Shell in Zig, or: Dave Learns How to Fork[3]

參考資料

[1] child_process.zig: https://github.com/ziglang/zig/blob/7dd1cf26f9b0cb104ada166a10ff356ca272577a/lib/std/child_process.zig#L1

[2] microHOWTO: Capture the output of a child process in C: http://www.microhowto.info/howto/capture_the_output_of_a_child_process_in_c.html

[3] Forking is Cool, or: A Unix Shell in Zig, or: Dave Learns How to Fork: https://ratfactor.com/zig/forking-is-cool

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